This invention relates generally to the measurement of the size distribution of gas clusters in gas cluster ion beams, and, more particularly to apparatus and methods for the measurement of the size distribution of gas clusters by making time-of-flight measurements.
The use of a gas cluster ion beam (GCIB) for etching, cleaning, and smoothing of material surfaces is known (see for example, U.S. Pat. No. 5,814,194, Deguchi et al.) in the art. For purposes of this discussion, gas clusters are nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such clusters typically consist of aggregates of from a few to several thousand molecules loosely bound to form the cluster. Such clusters can be ionized by electron bombardment or other means, permitting them to be formed into directed beams of known and controllable energy. The larger sized clusters are the most useful because of their ability to carry substantial energy per cluster ion, while yet having only modest energy per molecule. The clusters disintegrate on impact, with each individual molecule carrying only a small fraction of the total cluster energy. Consequently the impact effects of large clusters are substantial, but are limited to a very shallow surface region. This makes ion clusters effective for a variety of surface modification processes, without the tendency to produce deeper subsurface damage characteristic of monomer ion beam processing.
Means for creation of and acceleration of such GCIB""s are described in the reference (U.S. Pat. No. 5,814,194) previously cited. Presently available ion cluster sources produce clusters ions having a wide distribution of sizes, n (where n=the number of molecules in each clusterxe2x80x94in the case of monatomic gases like argon, an atom of the monatomic gas will be referred to as a molecule and an ionized atom of such a monatomic gas will be referred to as a molecular ionxe2x80x94or simply a monomer ionxe2x80x94throughout this discussion).
To a first order approximation, the surface modification effects of an energetic cluster is dependent on the energy of the cluster. However, second order effects are dependent on the velocity of the cluster which is dependent on both the energy of the cluster and it""s mass (and hence the cluster size, n.) In order to maximize the utility of a gas cluster ion beam for surface processing, it is useful to know and control both the energy of the cluster and the cluster size or cluster size distribution. In certain applications gas cluster ion beams are used for deposition or growth of surface films. When thus used, it is important to know the mass flow to the workpiece. The quantity of clusters is readily determined by measuring the cluster ion current that reaches the workpiece. In the usual case, ionized clusters from a practical ionized cluster source, do not necessarily all carry the same number of electrical charges. By suitable selection of ionization conditions, it can be arranged that the cluster ions predominately carry a single electrical charge, and in such case it is accurately assumed that each charge corresponds to a single cluster, but unless the average size or size distribution (average mass or mass distribution) is also known, the total mass flow to the workpiece is not known. It is also possible, by controlling the source conditions, to influence both the ratio of cluster ions to molecular ions and to influence the cluster size distribution as well. However, unless a means is available to measure and monitor the cluster size distribution and the ratio of cluster ions to molecular ions, meaningful adjustment and control of the source conditions for influencing ionized cluster size is difficult. When the ionized clusters do not all predominately carry a single charge or a known number of charges, knowledge of the ionized clusters"" mass per charge can also be used as a useful parameter to control or adjust the beam""s effectiveness for smoothing, etching, or other processing. For these and other reasons it is useful to have a means of measurement that can provide cluster size distribution information about a gas cluster ion beam or that can provide information about a gas cluster ion beam""s cluster-size-per-charge distribution or cluster-mass-per-charge distribution.
Because molecular ions, as well as cluster ions, are produced by presently available cluster ion beam sources, those molecular ions are accelerated and transported to the workpiece being processed along with the cluster ions. Molecular ions, having high energy with low mass results in high velocities, which allow the light molecular ions to penetrate the surface and produce deep damage which is likely to be detrimental to the process. Such sub-surface ion damage is well established and well known from the more traditional monomer ion beam processing art and can produce a variety of deep damage and undesirable implantation.
It has become known in the ion cluster beam art that many GCIB processes benefit from incorporating means within GCIB processing equipment for eliminating molecular ions from the ion cluster beams. Electrostatic (see U.S. Pat. No. 4,737,637, Knauer) and electromagnetic (see Japanese laid open application JP 03-245523, cited as prior art in U.S. Pat. No. 5,185,287, Aoyagi et al.) mass analyzers have been employed to remove light ions from the beam of heavier cluster ions. Electrostatic and electromagnetic mass analyzers have also been employed to select ion clusters having a narrow range of ion masses from a beam containing a wider distribution of masses.
Presently practical GCIB sources produce a broad distribution of ion cluster sizes with limited cluster ion currents available. Therefore it is not practical to perform GCIB processing by selecting a single cluster size or a narrow range of cluster sizesxe2x80x94the available fluence of such a beam is too low for productive processing. It is preferred to eliminate only the molecular ions and other lowest mass ions from the beam and use all remaining heavier ions for processing. Practical experience has shown that it is often sufficient to provide filtering to eliminate molecular ions while depending on the typical cluster size distribution characteristics (few small size clusters are created by typical sources) to limit the small clusters (n=2 to xcx9c10) in the beam. Clusters of size n greater than 10 are adequately large to provide acceptable results in most processes. Since the typical cluster distribution contains clusters of up to n=several thousand and there are few clusters of mass less than 100, it is not significantly detrimental if clusters up to size 100 are removed from the beam in the process of eliminating the molecular ions. However, in order to adequately predict the processing effectiveness of a gas cluster ion beam, it is very desirable to know the distribution of masses or cluster sizes in the beam and to know whether molecular ions and the smallest size cluster ions are present or not.
It is therefore an object of this invention to provide a way of measuring the mean cluster ion size or mass in a GCIB.
It is also an object of the invention to provide means to determine the cluster size (or mass) distribution or the mass flow of cluster ions in a GCIB processing system without necessitating the rejection of a portion of the beam through magnetic or electrostatic mass analysis.
It is also an object of the invention to determine the presence or absence in a GCIB of undesirable molecular ions.
The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow.
In the present invention it is preferably arranged that the molecular and cluster ions produced in an ionization system for gas clusters predominantly carry a single electrical charge and such ions are accelerated through a known acceleration potential. Based thereon, the ions, both molecular ions and cluster ions, in a GCIB produced in the apparatus of the invention have known and controllable energies per ion, regardless of cluster size or whether the ion is a cluster or a molecular ion. Since the ions are all generated from a pure gas, the molecular weight of each molecule is known. Therefore, by measuring the times-of-flight of the known ions of known energy over a known distance, it is possible according to the invention to calculate the cluster size distribution function for the ions in the beam. Alternatively, when it is not practical or not desired to arrange that all molecular and cluster ions produced in the gas cluster ionization system predominately carry a single electrical charge, in the present invention, by measuring the times-of-flight of the ions to calculate the cluster size per charge of the ions. The times-of-flight are determined by analyzing the fall-off of the GCIB current when the GCIB is abruptly terminated.
By providing a suitable beam gating method and introducing appropriate sensing apparatus in a conventional GCIB processor, the invention makes an in situ capability for measurement of the mass and cluster size distribution of the ions in a GCIB. The invention can measure either the mean cluster size (or mass) or the distribution of cluster sizes (or masses) in a GCIB.
For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims.